Na-23 nuclear magnetic resonance investigation of the lattice distortion in alpha-NaxV2O5

The full temperature dependence of the electric field gradient tensor at th e Na sites has been determined by nuclear magnetic resonance (NMR) in the t emperature range 8-330 K in alpha -Na,V2O5 (x = 0.996). Above the spin-Peie rls transition (T-c = 35.7 K), only a single Na site is observed in agreeme nt with the Pmmn space group proposed to describe this compound as the firs t example of a 1/4-filled ladder system. Below T-c, eight distinct quadrupo lar Na-23 sites are observed according to the distortion wave vector k(c) = (1/2, 1/2, 1/4) previously reported. In addition, the opening of a spin ga p is evidenced by a rapid drop of the magnetic hyperfine shift K-23 at T-c. The results are discussed in the context of a charge-order-driven spin-Pei erls transition.